1. Field of the Invention
The present invention relates to the field of hydraulic motors, and more particularly to a hydraulic motor drive apparatus capable of driving its rotary output shaft with a substantially constant horsepower over a wide speed range in one direction of shaft rotation.
Many machines require a rotary power drive that is capable of delivering substantially constant horsepower over a wide speed range. Such applications include metal cutting machine tools, for example, mills, drills, lathes, and so on; conveyor drives; material mixing drives; and others. The prime mover in these applications is generally an electric motor that runs at a near constant speed and is capable of delivering a maximum horsepower output. The desired load drive speed, however, can vary widely. In such applications it is desirable to provide a wide speed range, together with high efficiency so that the power output of the prime mover can be delivered to the load throughout the speed range.
For example, optimum metal removal in a milling machine will occur throughout a wide speed range, considering the machining of various metals and alloys, differing tool sizes and feed rates, and different tool types such as carbon steel, tungsten carbide, or other. Likewise, different machining operations, such as milling, drilling, reaming, and so on, require different speeds for optimum metal removal.
It is also evident that the capacity of a machine tool to do work will be related to the horsepower delivered to the cutting tool throughout the speed range demanded by optimum metal cutting considerations.
2. Description of the Prior Art
Conventional means for accomplishing substantially constant rotary power transmission over a wide speed range include: (1) a mechanical gear box with drive gear selection, (2) a multi-sheave belt drive, (3) a valve controlled hydraulic motor drive, and (4) a hydrostatic drive. Also, various combinations of these basic techniques have been used, such as hydrostatic drive plus mechanical gear selection.
Techniques (1) and (2) above generally suffer from the lack of continuous variable drive ratio selection. Split-sheave belt drives are an exception but these drives are generally slow in changing drive ratio, and are unwieldy in packaging for many applications.
Techniques (3) and (4) above offer continuous speed ratio change through use of a valve or other hydraulic control device such as a stroking device. Furthermore, either of these techniques lends itself to velocity control by use in a closed servo loop including a servovalve and an output drive speed sensor, such as a tachometer, for velocity feedback.
However, the disadvantage of a servovalve controlled hydraulic motor, technique (3) above, is that it involves severe inefficiency when operated over a wide speed range. This is due to the pressure drop across the servovalve associated with throttling the pump flow. This will be discussed more fully later herein. Various hydraulic pumping arrangements are used to reduce this inefficiency, such as variable displacement pumps, fixed displacement pump unloading circuits, accumulator discharge circuits, and others.
As to the hydrostatic drive, technique (4) above, it achieves excellent power efficiency as pump pressure builds to just what is necessary to move the load over and above the inefficiency of the drive components. However, a hydrostatic drive suffers from lack of drive stiffness whenever the hydraulic motor is located remote from the hydraulic pump. This is due to hydraulic compliance in the conduits for handling hydraulic drive fluid between the pump and motor. In applications, such as for the spindle drive of a milling machine, it is not convenient to mount the hydraulic pump and its driving electric motor close to the hydraulic spindle drive motor, as this would place all drive components on the head of the machine.
Another disadvantage of hydrostatic drives is that a single combination of pump and motor is required for each drive function. Thus in a milling machine, for example, if hydraulic table positioning drives are used, as with servovalves and actuators, and a hydrostatic spindle drive is desired, then a separate hydraulic pump is required for each drive system.